The advantages of powering lighting from a low voltage power supply are well known. Low voltage lighting is ‘softer’ and generally more preferable to high voltage lighting. The disadvantages of low voltage lighting are also well known. The main disadvantage is power loss in transmission cables linking a power supply with a lamp load. Lower voltage results in a higher current for a given power, resulting in greater power loss in the transmission cables. As well as the efficiency disadvantages of this, this creates control problems. In particular, the voltage drop across the transmission cables constitutes a significant proportion of the supply voltage. Consequently, it can be difficult to stabilise the supply voltage across the load at a required value from the power supply, which provides a source voltage. For the same reasons, it can be difficult to determine from the source voltage at the power supply what is the supply voltage across the light load. These problems apply also to the supply of low voltage power to loads other than light loads. The problems increase as the length of the transmission cables increases.
A known system 10 is illustrated in FIG. 1. Here, a power supply 11 provides a source voltage Vsource to a load 12 via a twin-core transmission cable 13. A voltage Vsupply across the load 12 is less than the power supply voltage Vsource by an amount equal to the voltage drop across the transmission cables 13 When the supply waveform Vsupply (which is shown in FIG. 2A, described below) across a resistive load 12 is subject to uncontrolled fluctuations due to uncontrolled changes in the power supply 11 (shown in FIG. 2C) or in the transmission of the Vsource waveform to the load 12, the RMS (root mean square) value of the Vsupply signal can destabilise. In this example, a 2 volt transmission-cable drop occurs between the peak of Vsource and the peak of Vsupply. This is known to be addressed in the prior art by providing appropriate alteration to the supply waveform Vsupply by controlled interventions in the transmission of the source waveform Vsupply to the load 12. This uses routing of Vsupply from the load to a detector, which provides information relating to Vsupply to an intervention controller, which controls intervention. This can stabilise the RMS value of the supply waveform Vsupply and thus stabilise the power output of the load 12, provided that the RMS value of the source waveform Vsource when attenuated by the transmission cables 13 is greater than the required stabilised value. This stabilisation is particularly useful for stabilising the light output of low voltage lamps, where fluctuations in voltage across the lamps due to changing volt drops in the transmission can present a problem.
In most cases involving stabilisation, the controlled intervention in the transmission of the source waveform Vsource to the load 12 is by an electronic switch, which temporarily interrupts the current to the load 12, thereby temporarily reducing the supply waveform voltage to zero or near zero, for a time necessary to reduce the RMS value of the supply waveform to the required stabilised level. This typically occurs in each half cycle of the V source waveform. The period of interruption for each half-cycle can be from the beginning of the half cycle until a phase-chopping angle at which conduction of current starts, as shown in FIG. 3A. This is known as leading edge dimming. Alternatively, interruption can occur from a phase-chopping angle at which conduction ceases until the end of the half cycle, as shown in FIG. 3B. This is known as trailing edge dimming. Alternatively, a combination of leading and trailing edge dimming can be used. In both leading and trailing edge technologies, the angular duration of the uninterrupted portion of the half-cycle, hereinafter called the conduction angle, is a measure which defines the angle of phase-chopping unambiguously. The RMS value increases as the conduction angle increases.
The Vsupply waveform is an example of a ‘detected waveform’ whose RMS value is required to be represented. The represented RMS value can be used to stabilise the waveform through interruption control, and can be used. for presentation. The detected waveform most commonly is sinusoidal AC (as in FIGS. 2A, 3A and 3B). As a precursor to applying the algorithm which generates this representation, this type of waveform is most conveniently rectified to produce a cyclically varying, positive unipolar ‘rectified waveform’, similar to the unipolar waveform U shown in FIGS. 2B and 3C. The algorithm acts on such a unipolar waveform U.
The rectified waveform may conveniently be derived from the AC detected waveform by full-wave bridge rectification, provided that the waveform voltage is high enough to make acceptable the inaccuracies necessarily caused by diode volt drops. A more accurate derivation is firstly to take one of the AC lines as ground so that the detected waveform is on the second AC line, to then generate by precision means an inversion (interchange of positive and negative values) of the detected waveform and finally use precision diode techniques to generate a “linear ‘OR’” function by which the rectified waveform follows whichever at any instant is the more positive of the detected waveform and the inversion waveform. Unipolar detected waveforms, such as those shown in FIGS. 2B and 2C, can be acted on directly by the algorithm. Both unipolar detected waveforms and unipolar rectified waveforms (hereinafter referred to as the ‘unipolar waveform’) are acted on by the algorithm. It will however be understood that other arrangements are possible, in particular those in which different positive and negative implementations of parts or all of the algorithm can act respectively on positive and negative half cycles of the detected AC waveforms. If leading-edge dimming has been used to modify Vsource, resulting in a waveform as shown in FIG. 3A, the unipolar waveform U will appear as in FIG. 3C.
In known algorithms, a combined waveform C is derived from the unipolar waveform. The instantaneous value of this combined waveform, represented in FIG. 4, is derived by taking whichever is the more positive of the value of the unipolar waveform U (of FIG. 3C) and the value of a plateau waveform P, which is shown in FIG. 5. Here, the waveform values are represented as analogue voltages. The combined waveform C of FIG. 4 may be generated simply using precision diode techniques to give the required “linear ‘OR’” function.
According to these algorithms, the mean value of the combined waveform C is taken as a representation of the RMS value of the detected waveform. This mean value clearly is not immediately available as an existing measurable value. Accordingly, a representation of the mean value (and thus of the RMS value), hereinafter called the ‘mean representation’ must be derived. There are numerous ways of automatically deriving the mean representation. The simplest mean representation is as a voltage value, referred to ground. This can be derived most simply by simple or multistage filtering of the combined waveform, which must be represented as a voltage waveform referred to ground, as in FIG. 4. Although this is the simplest, it has the downside of a lag in response caused by the filtering. The fastest derivation of the mean representation is achieved by deriving a value by integrating the combined waveform by analogue or digital means. The integration can be conducted every half cycle of a symmetric AC detected waveform, or every cycle of a unipolar or non-symmetric AC detected waveform.
The mean representation can, for stabilisation, be used to calculate or derive a correction to the conduction angle (or other controlled intervention in the transmission) for subsequent half cycles. For presentation, the mean representation can be used to control or update an analogue, digital or other presentation means of the RMS value. For stabilisation, an error signal is generated by comparing the mean representation with a set target level, representing the required stabilised level of the RMS value. This can be used to adjust the conduction angle or other parameter of a controlled intervention to the required level. For presentation, if a sufficiently constant mean representation is available over the whole of the half-cycle, it can control the presentation means directly. Otherwise, the error signal, generated by comparing the mean representation with the level of a store holding the presented RMS value, can be used to adjust the store to the new level of the mean representation.
A stabilising circuit is illustrated in FIG. 8A. Reference numerals are retained from FIG. 1 for like elements. In FIG. 8A, a power adjuster 23 is interposed close to the power supply 11 and in the circuit between the power supply 11 and the load 12. The power adjuster 23 interrupts the high (supply) current flowing from the power supply 11. Detector connection cables 20 carry Vsupply at a low current and substantially without attenuation to the power adjuster 23. The detector connection cables 20 thus provide the detected waveform. The power adjuster 23 has two other, low power, inputs, namely a plateau waveform P input and a set target level input 24. The output signal 28 of the power adjuster 23, which is an interrupted or phase-chopped version of Vsource, after attenuation by the transmission cables 13 becomes Vsupply.
FIG. 8B shows the power adjuster 23 in more detail. The detector connection cables 20 are connected to a rectifier 21, which provides precision rectification using diodes (not shown). The output of the rectifier 21 passes to a combiner 22, which combines the rectified unipolar signal U from the rectifier 21 with the plateau voltage P. The output of the combiner, C, (the combined waveform) is connected to a mean voltage calculator 25, which calculates the mean representation of the output of the combiner 22 in the manner described above, and provides a running mean signal at an output 26. An interrupt controller 27 receives the mean representation on the output 26, and uses it along with the set target level received at the set target level input 24 according to an algorithm to adjust the conduction angle such that Vsupply has the required RMS value.
The instantaneous value of the plateau waveform, hereinafter called the ‘plateau value’, is approximately constant over each half cycle, as shown in FIG. 5. However, the instantaneous value of the plateau waveform may be made to deviate from exact constancy in order to give a more accurate mean representation of the RMS value of detected waveforms conforming to certain shapes. For stabilisation, the instantaneous value of the plateau waveform is set at a value approximately equal to a factor of 0.78 times the value of the set target level. For presentation, the instantaneous value of the plateau waveform is set at a value, approximately equal to a factor of 0.78 times the value of the most recently derived mean representation. The plateau level is set at 0.78 times the value of the set target level because, at most phase-chopping angles, the average value of the combined waveform generated using this plateau level follows quite closely a multiple of the RMS value of the detected waveform. This multiple is close to but is not unity, and is compensated for by scaling either the set target (with plateau) level or the mean representation. If a zero plateau level were used, although there would be no fall-below window, the combined waveform would simply be a rectification of the detected waveform, meaning that its average value would follows the average value of the detected waveform instead of a multiple of its RMS value.
The plateau level setting factors may deviate from 0.78 for detected waveforms of certain shapes, for better matching of the mean representation to the RMS value of the detected waveform. The reasonably constant shape and value of the plateau waveform has been theoretically and empirically shown to give good matching for detected waveforms produced by phase-chopping Vsource waveforms approximating to a sine wave. For distorted waveforms modified by other types of intervention, other plateau waveforms may give the best matching.
In the most convenient arrangements where the waveforms are represented as analogue voltage waveforms referred to ground, the mean representation and any set target level are reasonably constant voltages referred to ground. Here, the reasonably constant plateau waveform P, similarly referred to ground, may be derived simply from whichever of the mean representation voltage and the set target level voltage is relevant, using an attenuating resistor network (not shown) returned to ground voltage.
A disadvantage with the above described algorithm is that changes in the unipolar waveform at values below the plateau waveform P value have no effect on the combined waveform C. For unipolar waveforms U whose shape when uninterrupted approximates a full-wave rectified sine wave, as shown in FIG. 6, the value of the unipolar waveform falls below the plateau waveform value P for periods at the beginning and end of each half cycle. During these ‘fall-below’ periods F, changes in phase-chopping angle are not detected. That is to say that if phase-chopping occurs within a fall below period, this has no effect on the resulting combined waveform and mean representation signal, since it is masked by the plateau waveform. In these instances, the mean representation may exhibit a small but significant deviation from the true RMS value. This deviation is called a ‘detection error’. It may be as large as the contribution made to the total RMS value of the uninterrupted supply waveform during the early and/or late sections of the half cycle during which the absolute value of this waveform falls below the plateau value, depending on whether leading- or trailing-edge, or both, types of dimming are used. FIG. 7 shows for a leading edge dimmed waveform the maximum error possible E with P and −P indicating respectively the plateau value and its inversion.